1. Field of the Invention
The present invention relates to a method allowing determination of a velocity model of seismic waves picked up by receivers coupled with an underground formation, from multi-offset records of these waves.
2. Description of the Prior Art
Seismic reflection surveys are widely used in petroleum exploration, notably to produce images of the subsurface from the information contained in the waves propagated and reflected from the geologic discontinuities of the subsoil.
More precisely, imagery methods use the kinematic information associated with the major seismic reflections (that is the travel times of the waves reflected on the main discontinuities of the subsoil) to determine a velocity macromodel of the subsoil, which will be used to convert the temporal seismic records to a depth image of the subsurface.
Access to the kinematic information necessary for determination of the velocity model requires interpretation of the seismic events in the seismic multi-offset records. Now, seismic multi-offset data is generally characterized by a bad signal-to-noise ratio, hence the failure of conventional automatic picking and the necessity of carrying out a long and costly manual picking of the seismic data. Besides, for 3D seismic surveys (currently predominant in relation to 2D surveys), the quantity of data to be interpreted is a 4D volume that can reach sizes of the order of one hundred gigabytes, or even of the order of a terabyte, which makes the interpretative task even longer and tedious.
In order to avoid this stage of seismic multi-offset collection interpretation, geophysicists have developed methods based on approximations of the geometry of the seismic events in the multi-offset collections. To establish these approximations, these methods put forward hypotheses on the subsoil complexity, hypotheses which can relate to the geometry of the geologic discontinuities of the subsoil and to the layer velocity variations. The method described by Taner and Kohler (1969) can for example be mentioned, which assumes a stratified medium of horizontal plane layers with homogeneous and isotropic layer velocities, as well as the method described by Levin (1971) which extends the previous method to sloping layers. Other variants have been proposed, but generally the existing methods are based on a hyperbolic hypothesis of the travel time curve in the multi-offset collections. This hypothesis is however violated from the moment that the geologic discontinuities of the subsoil are no longer plane and/or that the layer velocities exhibit lateral variations.